1. Field of the Invention
This invention concerns an apparatus and method by which the random acoustic vibrations emanated by a drill bit while drilling can be used to create seismic images of the subsurface in a region in the vicinity of the borehole. More particularly, it relates to such an apparatus and method in which sensors are positioned to provide more complete information on subterranean geologic formations. Most especially, it relates to such an apparatus and method in which improved reference signals and improved signal processing techniques are available.
2. Description of the Prior Art
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface acoustic impedance contrasts. The obtained image often suffers in spatial accuracy, resolution and coherence due to the long travel paths between source, reflector and receiver. It is well known to seismologists that the recorded signal amplitude at the receiver, A(r), is related to the input signal amplitude, A.sub.o, through the travel-path by: ##EQU1## where .alpha.=The absorption coefficient of the earth along the travel-path
r=The travel path length
Longer travel paths have lower recorded amplitudes.
A technique commonly known as Vertical Seismic Profiling (VSP) records seismic data to image the subsurface in the vicinity of the borehole. With VSP a source(s) is activated at the surface and a sensor(s) is suspended at a discrete borehole depth with a wireline. Data is recorded and the process repeated for several borehole depths. Acquiring VSP data is highly impractical if multiple surface source locations are desired. Each new source is costly to apply and adds to the acquisition time as well as the rig inactivity period. A Vertical Seismic Profile is therefore prohibitively expensive to implement when many source positions are used.
With a downhole source, the VSP geometry is inverted. Source locations become receiver locations and vice versa. A downhole source overcomes the logistical limitations inherent in using multiple surface sources for Vertical Seismic Profiling. Data can be acquired simultaneously at many sensor locations proximal to the Earth's surface with little more expense than a single location.
One of the earliest patents concerning downhole sources is Weatherby, U.S. Pat. No. 2,062,151, issued Nov. 24, 1936, which uses the drill bit as an impulse generator of seismic waves. Drilling is done with a cable tool, which is dropped on the hole bottom. This creates seismic impulses. Bit location and wave velocity can be obtained using these impulses. Drill bit generated direct wave arrival time differences between two non coincidental geophone locations are used to determine rock acoustic velocity. The Widrow U.S. Pat. Nos. 4,363,112 and 4,365,322, issued Dec. 7 and Dec. 21, 1982, use the continuous, natural, random vibrations of a rotary drill bit to launch seismic waves into the earth. Spectral amplitudes and interference patterns are used to image subsurface reflectors.
There are a number of previous patents concerned with the use of seismic downhole sources other than the drill bit. All of these patents make use of an artificial transducer situated at a given depth in the borehole. Broding et al., U.S. Pat. No. 3,909,776, issued September 1975 and Farr and Ward, U.S. Pat. No. 3,881,168, issued April 1975, use a fluid driven oscillator (as described by Galle in U.S. Pat. Nos. 3,520,362, 3,730,269, 3,860,902) to emit seismic waves into the earth from a position within a wellbore. Farr and Ward require that the seismic source be monofrequency. Phase delays between a geophone located on the surface near the well and another located near the top of the well are used to produce a log of travel-time and compressional wave velocity as a function of depth. Broding et al. use a fluid driven oscillator which changes emitted frequency as a function of time, much like a Vibroseis source, as described in U.S. Pat. No. 2,688,124. The repeatable pattern of the oscillator can then be used as a signal to cross-correlate with received geophone signals. Arrival times of events as determined by peaks in the cross-correlations can then be used to seismically image the subsurface.
The patent literature describes other types of downhole seismic sources which emit pulses into the subsurface. For example, Klaveness, U.S. Pat. No. 4,207,619, issued June 10, 1980, uses a pulse generator located in the drillstring just above the drill bit. The source is activated and the arrival time measured at the earth's surface and at the top of the drillstring on the swivel. The swivel location is used as a "zero point" sensor from which pulse arrival time differences can be measured. Arrival times are calculated from visual inspection of time-domain signals received by the geophones.
While the art pertaining to seismology is clearly a well-developed one, a need still remains for further development of it in order to meet the demands of gas, oil and other resource exploration.